Our efforts are currently focused on the biochemistry of DNA replication in mammalian cells: its initiation, synthesis and termination. Our approach is to develop assays for these three functions using isolated nuclei and subnuclear fractions extracted from SV40 infected cells. Such systems will allow control over biochemical parameters, purification, and characterization of DNA replication factors, and, most importantly, complementation of cell and virus mutants in vitro, a prerequisite to identifying gene products. BIBLIOGRAPHIC REFERENCES: DePamphilis, M.L., Beard, P., and Berg, P. "Synthesis of covalently closed, superhelical Simian Virus 40 DNA in cell lysates", J. Biol. Chem. 250: 4340-4347 (1975). DePamphilis, M.L., and P. Berg. "Requirement of a cytoplasmic fraction for synthesis of SV40 DNA in isolated nuclei", J. Biol. Chem. 250: 4348-4354 (1975).